Chap. V. ACCLIMATISATION. 141 



fertile (a far severer test) under them, may be used as 

 an argument that a large proportion of other animals, 

 now in a state of nature, could easily be brought to bear 

 widely different climates. We must not, however, push 

 the foregoing argument too far, on account of the pro- 

 bable origin of some of our domestic animals from seve- 

 ral wild stocks: the blood, for instance, of a tropical 

 and arctic wolf or wild dog may perhaps be mingled in 

 our domestic breeds. The rat and mouse cannot be 

 considered as domestic animals, but they have been 

 transported by man to many parts of the world, and 

 now have a far wider range than any other rodent, 

 living free under the cold climate of Faroe in the 

 north and of the Falklands in the south, and on many 

 islands in the torrid zones. Hence I am inclined to look 

 at adaptation to any special climate as a quality readily 

 grafted on an innate wide flexibility of constitution, 

 which is common to most animals. On this view, the 

 capacity of enduring the most different climates by man 

 himself and by his domestic animals, and such facts as 

 that former species of the elephant and rhinoceros were 

 capable of enduring a glacial climate, whereas the liv- 

 ing species are now all tropical or sub-tropical in their 

 habits, ought not to be looked at as anomalies, but 

 merely as examples of a very common .flexibility of con- 

 stitution, brought, under peculiar circumstances, into 

 play. 



How much of the acclimatisation of species to any 

 peculiar climate is due to mere habit, and how much to 

 the natural selection of varieties having different innate 

 constitutions, and how much to both means combined, is 

 a very obscure question. That habit or custom has some 

 influence I must believe, both from analogy, and from 

 the incessant advice given in agricultural works, even 

 in the ancient Encyclopaedias of China, to be very cau- 



